Method of and apparatus for solid state imaging device

Abstract: There is provided a method of driving a solid state imaging device (51) in which an imaging section (54) for converting incident light into a signal charge and transferring the signal charge and an accumulation section (56) for temporarily accumulating the signal charge from the imaging section and transferring the same. This method is characterized in that a transfer clock pulse ( ST) applied to a vertical transfer register (55) in the accumulation section and fixing a potential within the vertical transfer register (55) to a potential deeper than a potential of low level of the vertical transfer register (53) in the imaging section to thereby discharge and transfer excess electric charges. Also, a solid state imaging device (51) comprises an imaging section (54) composed of a plurality of sensor sections (52) arranged in horizontal and vertical directions and first vertical transfer section (53) disposed in response to every sensor section arranged in the vertical direction and sequentially transferring signal charges read out from each of the sensor sections in the vertical direction by movement of potential wells, a storage section (56) having second vertical transfer sections (55) disposed in response to the first vertical transfer sections (53), receiving signal charges transferred from the first vertical transfer sections by movement of the potential well and temporarily storing the signal charge, a horizontal transfer section (57) for reading signal charges transferred thereto from the second vertical transfer section (55) at a predetermined timing and transferring the same in the horizontal direction, a signal read-out section (59) for reading signal charges sequentially transferred from the horizontal transfer section, and a smear drain section (58) for discharging unnecessary electric charges of the first and second vertical transfer sections, wherein there is provided a coarse unnecessary-electric-charge discharge period in which a potential level of at least one of the first and second vertical transfer sections is arranged to be flat in the transfer direction by the application of a vertical gate clock voltage.

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to solid state imaging devices and, more particularly, is directed to an FIT (frame interline transfer) type solid state imaging device in which signal charges read out from a sensor section are transferred by a vertical register to a storage section and temporarily stored therein and a driving method thereof. Description of the Prior Art: Heretofore, FIT (frame interline transfer) type solid state imaging devices are known as CCD (charge-coupled device) solid state imaging devices. FIG. 1 of the accompanying drawings shows an overall arrangement of a typical FIT type CCD solid state imaging device 1. FIG. 2 is a timing chart of clock pulses for driving the CCD solid state imaging device 1 and a vertical blanking pulse V-BLK in which only vertical transfer clock pulses IM 1 , ST 1 for driving the imaging section 4 and vertical transfer registers 3, 5 of the accumulation section 6 are illustrated as the clock pulses. Other vertical transfer clock pulses IM 2 , IM 3 , IM 4 , ST 2 , ST 3 , ST 4 are substantially similar to the vertical transfer clock pulses  IM 1 , ST 1 and therefore need not be described. As shown in FIG. 1, the FIT type CCD solid state imaging device 1 includes an imaging section 4 formed of a number of sensor sections 2 which serve as pixels arranged in a two-dimensional fashion and vertical transfer registers 3 having CCD structures provided on one side of each sensor section column, an accumulation section 6 formed of a plurality of vertical transfer registers 5 having CCD structures in response to the vertical transfer registers 3 of the imaging section 4, a horizontal transfer register 7 having a CCD structure, a drain region 8 provided on the opposite side of the accumulation section 6 of the horizontal transfer register 7 through a gate section 10 for discharging excess electric charges, and a signal charge detecting section 9 connected to the final stage of the horizontal transfer register 7. The vertical transfer registers 3 in the imaging section 4 are driven by 4-phase vertical transfer clock pulses IM 1 , IM 2 , IM 3 , IM 4 , and the vertical transfer registers 5 in the accumulation section 6 are driven by 4-phase vertical transfer clock pulses ST 1 , ST 2 , ST 3 , ST 4 . Further, the horizontal transfer register 7 is driven by 2-phase horizontal transfer clock pulses H 1 , H 2 . A clock pulse  D is applied to the gate section 10 and a DC voltage V D is applied to the drain region 8. A read-out operation of the FIT type CCD solid state imaging device 1 is carried out as follows. Initially, during a vertical blanking period 11, signal charges photoelectrically-converted by the respective sensor sections 2 are read out from the sensor sections 2 to the vertical transfer registers 3. The signal charges are then transferred from the vertical transfer registers 3 in the imaging section 4 to the vertical transfer registers 5 in response to a frame shift transfer pulse 13. Signal charges of every horizontal line are transferred from the vertical transfer registers 5 to the horizontal transfer register 7 in response to a line shift transfer pulse 14, whereafter the signal charges are transferred within the horizontal transfer register 7 in response to the 2-phase transfer clock pulses H 1 , H 2 and then outputted from the signal charge detecting section 9. Thereafter, excess electric charges remaining in the vertical transfer registers 3 in the imaging section 4 are discharged to the accumulation section 6 in response to an excess electric charge discharging and transferring pulse 15. The excess electric charges are discharged from the accumulation section 6 to the drain region 8 through the gate section10. Then, signal charges are again read out from the sensor sections 2 in response to the read-out pulse 12. The above-mentioned operation will hereinafter be repeated and the read-out operation of the FIT type CCD solid state imaging device 1 is carried out. The FIT type CCD solid state imaging device 1 needs a frame shift transfer operation and an excess electric charge discharging and transferring operation as compared with the normal IT (interline transfer) type CCD solid state imaging device. Therefore, the FIT type CCD solid state imaging device 1 consumes an electric power of about three times as large as the power consumption of the IT type CCD solid state imaging device in order to drive the vertical transfer registers 3, 5. Moreover, since it is customary that the FIT type CCD solid state imaging device 1 is used in camera having a large optical system (1-inch or 2/3-inch) to obtain a high picture quality as compared with the IT type CCD solid state imaging device, a chip area of the FIT type CCD solid state imaging device 1 is increased. As a consequence, a power consumption of the FIT type CCD solid state imaging device 1 tends to increase. The increase of the power consumption is not an easy problem to be solved because a temperature of the CCD imaging device rises when the power consumption is increased. Therefore, the rise of temperature should be suppressed. There is then the problem that a camera portion in the CCD solid state imaging device cannot be miniaturized. These problems become serious particularly when a large camera, such as for an HDTV (high definition television) receiver, is used for ENG (electric news gathering). Further, it is known that electric charges are generated when a charge-coupled device forming a charge transfer section senses light and that electric charges cause a false signal in the output signal of the solid state imaging device. The electric charge and the false signal are called a smear electric charge and a smear false signal, respectively. As shown in FIG. 1, the imaging section 4 and the storage section 6 include a plurality of vertical registers 3,5 forming the vertical transfer section. Signal charges generated by the sensor sections 2 in the imaging section 4 are transferred through the vertical registers 3,5 and the horizontal register 7 sequentially and outputted from the signal charge detecting section 9. Smear electric charges produced in the vertical registers 3 are discharged by the smear drain section 8. FIG. 3 is a timing chart of transfer clocks for controlling a transfer driver which drives the vertical register 3 in the conventional CCD solid state imaging device shown in FIG. 1. Drivers that drive the vertical registers 3,5 of the imaging section 4 and the storage section 6 are respectively driven by 4-phase transfer clocks  IM 1 to IM 4 and ST 1 to ST 4 . Signal charges that are accumulated in the storage section 6 are outputted in response to a line shift transfer clock 21 and then a smear discharging and transferring 22 for discharging smear electric charges from the vertical register 3 is carried out. The smear discharging and transferring 22 is carried out at an extremely high speed because the read-out cycle of the signal charge should be made at high speed. Then, new signal charges are read out from the vertical register 3 by a signal charge read-out 23. The signal charges thus read out are transferred by a frame shift transfer 24 from the imaging section 4 to the storage section 6, thereby being accumulated temporarily. Signal charges are transferred at every line from the horizontal register by a line shift transfer 25 and outputted through the signal charge detecting section 9. When an object of high brightness is picked up by the conventional image sensor, a smear false signal 45 caused by smear electric charges 41 (see FIG. 5) is reproduced on the upper side of the monitor screen and the quality of picture reproduced on the monitor picture screen is deteriorated because it has been impossible in the conventional image sensor to discharge smear electric charges under necessary and sufficient condition. This problem will be described with reference to FIG. 4. FIG. 4 show a schematic structure of a cross section taken through the line Y1 - Y2 in FIG. 1, and potential levels and charge transfer states at respective positions in the cross-section of the conventional image sensor. As shown in FIG. 4, at a timing point tl in which the line shift transfer was ended, much smear electric charges 41 that had been generated by some causes, such as illumination of high brightness, are accumulated in vertical registers 3, 5. Then, the smear discharging and transferring 22 is carried out in order to discharge the smear electric charges 41 accumulated in the vertical registers 3, 5. Since it is customary that the smear discharging and transferring 22 is carried out at high speed during an extremely short period of time, it is difficult to discharge the smear electric charges 41 completely. In particular, when there are much smear electric charges 41, the smear electric charges 41 cannot be discharged completely and the smear electric charges 41 of an appreciable amount are left in the vertical registers 3 of the imaging section 4 even at a timing point t2 after the discharging and transferring 22 was ended. In this state, if a read-out 23 of new signal charges from the sensor section 2 is carried out (at a timing point t3), then signal charges 43 and the smear electric charges 41 are mixed with each other. Mixed electric charges are transferred through a storage section 6 and the horizontal register 7 by a frame shift transfer 24 and a line shift transfer 25 and outputted from the signal charge detecting section 9. As a consequence, as shown in FIG. 5, the smear false signal caused by the smear electric charge 41 is reproduced on the upper side of the monitor picture screen and the quality of a reproduced picture is deteriorated. Moreover, the amount of smear electric charges is increased as the brightness of object is increased with the result that a lot of smear false signals are generated. When there are much smear electric charges, the vertical register needs a large driving power to transfer electric charges and the power consumption of the solid state imaging device is increased. OBJECTS AND SUMMARY OF THE INVENTION It is therefore a first object of the present invention to provide a method of driving an FIT type solid state imaging device in which a power consumption can be reduced. It is a second object of the present invention to provide a solid state imaging device in which the occurrence of a smear false signal can be suppressed even when an object of high brightness is picked up and in which a power consumption required to discharge smear electric charges can be reduced. In order to attain the first object of the present invention, there is provided a method of driving a solid state imaging device which includes an imaging section for converting incident light into a signal charge and transferring the signal charge and an accumulation section for temporarily accumulating the signal charge from the imaging section and transferring the same. This method of driving a solid state imaging device is characterized in that a transfer clock pulse applied to a vertical transfer register in the accumulation section and fixing a potential within the vertical transfer register to a potential deeper than a potential of low level of the vertical transfer register in the imaging device to thereby discharge and transfer excess electric charges. Advantageously, transfer clock pulses applied to said vertical transfer register in said accumulation section are temporarily stopped to fix said vertical transfer register to a constant potential deeper than a potential of low level of said vertical transfer register in said imaging section to thereby discharge and transfer excess electric charges in a substantially continuous manner. According to the present invention, during the discharge and transfer period of excess electric charges generated in the imaging section, excess electric charges are discharged and transferred under the condition that the transfer clock pulses applied to the vertical transfer register in the accumulation section are stopped, accordingly, the voltage is fixed and that the potential provided within the vertical transfer register is fixed to be a potential deeper than a low level potential of the vertical transfer register in the imaging section. Therefore, the power consumption can be reduced. Specifically, transfer clock pulses for discharging and transferring excess electric charges are applied to the vertical transfer register in the imaging section and the excess electric chargcs are discharged to the vertical transfer register of the accumulation section. The vertical transfer register in the accumulation section is fixed to the constant potential and the excess electric charges that were discharged to the accumulation section are discharged and transferred to the drain region in a so-called continuous manner (may be based on a self-induction drift and thermal diffusion mode). Accordingly, the power consumption for discharging and transferring the excess electric charges in the accumulation section become substantially zero and the power consumption for driving the whole of the vertical transfer registers is reduced. In this case, if the potential provided within the vertical transfer register in the accumulation section is fixed to be constant, then the excess signal charges in the accumulation section can be transferred in a continuous manner more smoothly. In view of an aspect that smear electric charges are coarsely discharged during the early stage of the smear electric charge discharge and transfer period so that the potential of the vertical register in the imaging section should be flat, the present invention is made. The invention further relates to a method of driving a solid state imaging device which includes: an imaging section composed of a plurality of sensor sections arranged in horizontal and vertical directions and first vertical transfer section disposed in response to every sensor section arranged in said vertical direction and sequentially transferring signal charges read out from each said sensor section in the vertical direction by movement of potential wells; a storage section having second vertical transfer sections disposed in response to said first vertical transfer sections, receiving signal charges transferred from said first vertical transfer sections by movement of said potential well and temporarily storing said signal charges; a horizontal transfer section for reading signal charges transferred thereto from said second vertical transfer sections at a predetermined timing and transferring the same in the horizontal direction; a signal read-out section for reading signal charges sequentially transferred from said horizontal transfer section; and a smear drain section for discharging unnecessary electric charges of said first and second vertical transfer sections, characterized in that there is provided a coarse unnecessary-electric-charge discharge period in which a potential level of at least one of said first and second vertical transfer sections is arranged to be flat in the transfer direction by the application of a vertical gate clock voltage. In the coarse discharge period used by the solid state imaging device and the driving method thereof according to the present invention, the potential levels of the vertical transfer section are arranged to be flat in the transfer direction by the application of the vertical gate clock voltage in the vertical transfer section in which electric charges are transferred by moving the potential wells in the transfer direction, whereby a large quantity of smear electric charges are coarsely discharged one at a time without the transfer clock. According to the solid state imaging device of the present invention, this smear electric charge coarse discharge period should preferably be accompanied with the succeeding smear electric charge discharge and transfer period. However, the accompaniment of the succeeding smear electric charge discharge and transfer period is not indispensable for the present invention. According to other preferred embodiment of the present invention, the smear drain section is disposed adjacent to the second vertical transfer section. The potential level of the first vertical transfer section is held at low level and the potential level of the second vertical transfer section is held at high level in the transfer direction, respectively. Then, the smear electric charges are discharged to the smear drain section from the first vertical transfer section through the second vertical transfer section. Alternatively, according to a specific embodiment of the present invention, there may be provided the first and second smear drain sections corresponding to the first and second vertical transfer sections. During the coarse discharge period, only the potential level of the first vertical transfer section is arranged to be flat and the second vertical transfer section may carry out the line shift transfer, for example. There is then the advantage that the coarse discharge period can be extended. Specifically, according to this further embodiment, the invention relates to a method of driving a solid state imaging device comprising: an imaging section composed of a plurality of sensor sections arranged in horizontal and vertical directions and first vertical transfer sections disposed in response to every sensor section arranged in said vertical direction and sequentially transferring signal charges read out from each said sensor section in the vertical direction by movement of potential wells; a storage section having second vertical transfer sections disposed in response to said first vertical transfer sections, receiving signal charges transferred from said first vertical transfer sections by movement of said potential wells and temporarily storing said signal charges; a horizontal transfer section for reading signal charges transferred thereto from said second venical transfer section at a predetermined timing and transfemng the same in the horizontal direction; a signal read-out section for reading signal charges sequentially transferred from said horizontal transfer section; a first smear drain section for discharging unnecessary electric charges from said first and second vertical transfer sections; and a second smear drain section for discharging unnecessary electric charges from said first and second vertical transfer sections; characterized in that there is provided a coarse unnecessary-electric-charge discharge period in which a potential level of at least one of said first and second vertical transfer sections is arranged to be flat in the transfer direction by the application of a vertical gate clock voltage. According to the driving method of a solid state imaging device of the present invention, since the smear electric charge generated in the vertical transfer section can be freely moved in the vertical transfer section of the same potential level by arranging the potential level of at least one of the vertical transfer sections to be flat in the transfer direction, the electric charges are transferred without the clock signal and at least one portion of electric charges is discharged to the smear drain section one at a time. Therefore, the rapid coarse discharge of smear electnc charges becomes possible. Then, when smear electric charges are discharged and transferred, it is sufficient that only a small quantity of smear electric charges that were left after the coarse discharge may be transferred. Therefore, the high speed discharge and transfer does not need a large driving force and the discharge of smear electric charge becomes possible. The above and other objects, features, and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof to be read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a structure of a conventional FIT type CCD solid state imaging device; FIG. 2 is a timing chart of transfer clock pulses according to a conventional driving method; FIG. 3 is a timing chart showing timings of electric charge transfer clocks used in the conventional solid state imaging device; FIG. 4 is a schematic diagram showing a crosssectional structure of the vertical register and potential levels of that portion and the transferred states of smear electric charges; FIG. 5 is a front view of a monitor used to explain a smear false signal produced from an output of a conventional solid state imaging device; FIG. 6 is a diagram showing a structure of a typical FIT type CCD solid state imaging device which is applied to the present invention; FIG. 7 is a timing chart of transfer clock pulses showing a driving method according to a first embodiment of the present invention; FIG. 8 is a cross-sectional view taken along the line Y - Y in FIG. 6; FIG. 9 is a potential diagram showing electric charge transferred states at every timings t21 to t24 according to the first embodiment; FIG. 10 is a table showing power consumptions according to the embodiment of the present invention; FIG. 11 is a timing chart of transfer clock pulses showing a driving method according to a second embodiment of the present invention; FIG. 12 a potential diagram showing electric charge transferred states at every timings t21 to t24 according to the second embodiment of the present invention; FIG. 13 is a table showing power consumptions in the second embodiment of the present invention; FIG. 14 is a diagram of waveforms of vertical transfer clock pulses IM 1 to IM 4 and ST 1 to ST 4 ; FIG. 15 is a schematic plan view showing an FIT type solid state imaging device including a solid state imaging device according to a third embodiment of the present invention; FIG. 16 is a timing chart showing examples of timings of charge transfer clocks in the solid state imaging device according to the third embodiment of the present invention; FIG. 17 is a schematic diagram showing a crosssectional structure of the vertical register in the third embodiment of the present invention, potential levels at that portion and transferred states of smear electric charges; FIG. 18 is a plan view similar to FIG. 15 and showing a solid state imaging device according to a fourth embodiment of the present invention; FIG. 19 is a timing chart similar to FIG. 16 and to which reference will be made in explaining operation of the solid state imaging device shown in FIG. 18; and FIG. 20 is a diagram similar to FIG. 17 and to which reference will be made in explaining operation of the solid state imaging device shown in FIG. 18. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method of driving an FIT type CCD solid state imaging device according to an embodiment of the present invention will be described with reference to the drawings. FIG. 6 is a diagram showing an overall arrangement of a typical FIT type solid state imaging device to which a driving method according to a first embodiment of the present invention is applied. Similarly to FIG. 1, the FIT CCD solid state imaging device comprises an imaging section 54 having a number of sensor sections 52 serving as pixels arranged in a two-dimensional fashion and vertical transfer registers 53 having CCD structure provided on one side of sensor section columns, an accumulation section 56 composed of a plurality of vertical transfer registers 55 formed of CCDs corresponding to the vertical transfer registers 53 in the imaging section 54, a horizontal transfer register 57 formed of CCD, a drain region 58 disposed adjacent to the horizontal transfer register 57 on the opposite side of the accumulation section 56 through a gate section 60 and a signal charge detecting section 59 connected to the final stage of the horizontal transfer register 57. The vertical transfer registers 53 in the imaging section 54 are driven by 4-phase vertical transfer clock pulses IM 1A , IM 2A , IM 3A , IM 4A , and the vertical transfer registers 55 in the accumulation section 56 are driven by 4-phase vertical transfer clock pulses ST 1A , ST 2A , ST 3A , ST 4A . The horizontal transfer register 57 is driven by 2-phase horizontal transfer clock pulses H 1A , H 2A . A clock pulse  DA is applied to the gate section 60, and a DC voltage V DA is applied to the drain region 58. FIG. 7 is a timing chart of the vertical transfer clock pulses IM 1A , IM 2A , IM 3A , IM 4A , ST 1A , ST 2A , ST 3A , ST 4A and a vertical blanking pulse V-BLK. FIG. 14 shows diagrams of waveforms of the transfer clock pulses  IM, ST used when signal charges are transferred by the vertical transfer registers 53, 55. FIG. 8 is a cross-sectional view taken along the line Y - Y in the CCD solid state imaging device 51 shown in FIG. 6. FIG. 9 is a potential diagram showing at every timing the transferring of electric charges at the above cross section according to the driving method of FIG. 7. As shown in a cross-sectional view of FIG. 8, a second conductivity type, i.e., P type well region 92 is formed on a first conductivity type, for example, N type silicon semiconductor substrate 91, and an N type transfer channel 93 is formed on the P type well region 92. A vertical transfer register 53 is formed by arranging a plurality of transfer electrodes 95 applied with the transfer clock pulses IM 1A ,  IM 2A , IM 3A , IM 4A through a gate insulating film on a transfer channel region 83 corresponding to the imaging section 54. A vertical transfer register 55 is formed by arranging a plurality of transfer electrodes 96 applied with the transfer clock pulses  ST 1A , ST 2A , ST 3A , ST 4A through a gate insulating film on the transfer channel region 85 corresponding to the accumulation section 56. A horizontal transfer register 57 is formed of an array of a plurality of transfer electrodes 97 applied with the transfer clock pulses H 1A , H 2A on the corresponding part 87 of the transfer channel region 93 through the gate insulating film in the direction perpendicular to the sheet of drawing of FIG. 9. The drain region 88 is formed of an N+ diffusion region 98 and applied with a DC voltage V DA . The gate section 60 is arranged by forming a gate electrode 99 applied with a clock pulse  DA on the part 90 of the region 93 provided between the N+ diffusion region 98 and the part 87 corresponding to the horizontal transfer register 57 through the gate insulating film. A read-out operation according to the first embodiment of the present invention will be described with reference to FIGS. 6 to 9. Initially, at a timing point t21 in the vertical blanking period, signal charges 101 that were photoelectrically-converted by the sensor sections 52 are read out from the sensor sections 52 to the vertical transfer register 53 in response to a read-out pulse 71. The signal charges thus read out are transferred to the under portions of the first and third transfer electrodes 95 to which there are applied the clock pulses  IM 1A , IM 3A (see FIG. 9A). Though not shown, signal charges 101 of corresponding two lines will be mixed and transferred in response to odd and even fields of the interlace scanning. Subsequently, the signal charge 101 is transferred from the imaging section 54 to the vertical transfer register 55 in the accumulation section 56 at high speed in response to the frame shift transfer pulse 72 in the vertical transfer clock pulses IM 1A to IM 4A and ST 1A to ST 4A . FIG. 9B shows the state that signal charges are transferred at a timing point t22 during the frame shift transfer period T 1 . The signal charges 101 are transferred at every horizontal line to the horizontal transfer register 57 in response to a line shift transfer pulse 73 in the vertical transfer clock pulses IM 1A to IM 4A and ST 1A to ST 4A , whereafter the signal charges 101 are transferred within the horizontal transfer register 57 in response to the horizontal transfer clock pulses H 1A , H 2A and outputted from the signal charge detecting section 59. FIG. 9C shows the state that signal charges are transferred at a timing point t23 during the line shift transfer period T 2 . The gate section 60 is held at high level potential during the period T 2 . Thereafter, excess electric charges 102, such as a smear component remaining within the imaging section 54 and a dark current component, are discharged to the accumulation section 56. The excess electric charges 102 are discharged to the drain region 58 from the accumulation section 56 through the horizontal transfer register 57 and the gate section 60. The signal charges 101 are read out from the sensor section 52 in response to the read-out pulse 71 one more time. The read-out operation of the FIT type CCD solid state imaging device is carried out by repeating the above-mentioned operation. According to the first embodiment of the present invention, during the excess electric charge discharging and transferring period T 3 , although the imaging section 54 makes the vertical transfer clock pulses IM 1A to IM 4A , accordingly, the excess electric charge discharging and transferring pulse 74, the accumulation section 56 disables the vertical transfer clock pulses ST 1A to ST 4A and fixes these clock pulses  ST 1A to ST 4A at high level voltage, whereby the potential (so-called channel potential) within the vertical transfer register 55 in the accumulation section 56 are held at low level, thereby making it possible to reduce the power consumption. During the excess electric charge discharging and transferring period T 2 , the vertical transfer register 53 in the imaging section 54 discharges the excess electric charges 102 to the accumulation section 56 in response to the high speed transfer clock pulse for discharge and transfer excess electric charges, i.e., excess electric charge discharging and transferring pulse 74. The vertical transfer register 55 in the accumulation section 56 discharges and transfers the excess electric charges 102 freely. It is considered that this free transferring is mainly based on a self-induction drift and a thermal diffusion mode. Inasmuch as this free transferring is slow in electric charge transferring speed as compared with the transferring using the high speed transfer clock pulse in the imaging section 54, i.e., the excess electric charge discharging and transferring pulse 74, it is expected that much excess electric charges remain in the accumulation section 56. However, even though much excess electric charges 102 remain in the accumulation section 56, it is sufficient that the excess electric charges 102 may be transferred from the imaging section 54 sufficiently. That is, according to this embodiment, the excess electric charge 102 is discharged in the imaging section 54 by the high speed transfer clock 74, which causes no trouble in actual practice. Even when the excess electric charge 102 remains in the accumulation section 56, the remaining excess electric charge is discharged in the drain region 58 in the succeeding frame shift transfer. There is then no problem. In the first embodiment, the power consumption for driving the vertical transfer registers 53, 55 is roughly estimated as follows. As shown in FIG. 10, while the power consumptions required in the transfer of electric charges in the imaging section 54 and the accumulation section 56 are almost the same as each other, according to this embodiment, the power consumption required when the excess electric charges are discharged in the accumulation section 56 become almost zero. Therefore, the power consumption that is required to drive the vertical transfer registers 53, 55 is reduced to about 5/6. While the vertical transfer register is driven by the 4-phase driving mode as described above, the present invention is not limited thereto and can be applied to a vertical transfer register that is driven under other driving modes, such as a three-phase driving system or the like. Further, while the clocks of the transfer pulses ST 1A to ST 4A in the accumulation section 56 are fixed and held at high level voltages during all excess electric charge transfer period T 3 as described above, these clock pulses ST 1A to ST 4A are not always fixed and held at high level voltages. The transfer pulses ST 1A to ST 4A of the accumulation section 56 may be stopped and held at high level voltage at least in part of the excess electric charge discharging and transferring period T 3 with objects of the present invention being achieved to some extent. The driving method according to the second embodiment of the present invention will be described below. According to this embodiment, in addition to the first embodiment, if the clocks of the line shift transfer pulses in the vertical transfer clock pulses IM 1A to IM 4A of the imaging section 54 are stopped and then held at a constant voltage, then the power consumption reducing effect can be enhanced more. FIG. 11 is a timing chart of vertical transfer clock pulses IM 1A , IM 2A , IM 3A , IM 4A , ST 1A , ST 2A , ST 3A , ST 4A and vertical blanking pulse V-BLK according to the second embodiment of the present invention. FIG. 12 is a potential diagram showing the condition that electric charges are transferred by the driving method shown in FIG. 11 at every timings t21 to t24. According to this embodiment, during the excess electric charge discharging and transferring period, the clocks of the transfer pulses ST 1A , ST 2A ,  ST 3A , and ST 4A in the accumulation section 56 are stopped and fixed to high level voltages, thereby setting the potential produced within the vertical transfer register in the accumulation section at low level (see FIG. 12D). Further, during the line shift transfer period T 2 , the clocks of the vertical transfer pulses IM 1A , IM 2A , IM 3A , IM 4A are stopped and fixed at constant voltages (low level voltages), thereby setting the potential provided within the vertical transfer register 53 in the imaging section 54 to be constant (see FIG. 12C). According to this driving method, the excess electric charge discharging pulses in the vertical transfer clock pulses ST 1A to ST 4A in the accumulation section 56 can be omitted and the line shift transfer pulses in the vertical transfer clock pulses IM 1A , IM 2A , IM 3A and IM 4A in the imaging section 54 also can be omitted, thereby reducing the power consumption more. Even when the line shift transfer of the imaging section is stopped, the signal charge 101 is transferred in the accumulation section 56 as shown in FIG. 12C. There is then no problem. Further, if the potential produced within the vertical transfer register 53 in the imaging section 54 during the line shift transfer period T 2 is arranged to be high level as described above, then the excess electric charge 102 produced in the imaging section 54 can be constantly discharged to the accumulation section 56 so that much of the excess electric charges 102 can be prevented from being left in the imaging section 54. According to the second embodiment of the present invention, the power consumption for driving the whole of the vertical transfer registers 53, 55 can be roughly estimated as follows. As shown in FIG. 13, while the power consumptions in the respective transfers in the imaging section 54 and the accumulation section 56 are all approximately the same according to the prior art, according to this embodiment, the power consumption required to discharge and transfer excess electric charges in the accumulation section 56 and the power consumption required to effect the line shift transfer in the imaging section 54 become substantially zero. Accordingly, the power consumption necessary for driving the vertical transfer registers 53, 55 is reduced to about 2/3. While the vertical transfer clock pulses in the imaging section 54 are stopped during the whole of the line shift transfer period T 2 as described above, the vertical transfer clock pulses are not always stopped. If the vertical transfer pulses IM 1A to IM 4A in the imaging section 54 are stopped and then fixed to low level voltages at least in one portion of the line shift transfer period T 2 , then the objects of the present invention can be attained to some extent. According to this embodiment, if the transfer pulses ST 1A to  ST 4A of the accumulation section 56 are stopped and fixed to high level voltages during one portion of the excess electric charge discharge and transfer period T 3 of the whole of the excess electric charge discharge and transfer period T 3 , then the objects of the present invention can be achieved to some extent. While the vertical transfer register is driven under the 4-phase driving mode as described above, the present invention is not limited thereto and the present invention can similarly be applied to a vertical transfer register that is driven under other driving modes, such as the 3-phase driving mode or the like. While the potentials of the vertical transfer registers in the accumulation section 56 are set to be constant (low) potential and potentials produced within the vertical transfer register 55 are all set to be potentials of constant level when excess electric charges are discharged and transferred according to the first and second embodiments of the invention, the present invention is not limited thereto and the following variant is made also possible. That is, potential levels within other vertical transfer registers 55 are not always made uniform. Specifically, if the transfer pulses  ST 1A to ST 4A are stopped such that the potential within the vertical transfer register 55 is set to be deeper than the potential of low level of the vertical transfer register 53 in the imaging section 54, then the excess electric charges can be discharged. In this case, if the potentials produced within the vertical transfer register 55 are all set to be constant level potential, then excess electric charges can be discharged more smoothly. According to the present invention, it is possible to reduce the power consumption of the FIT type CCD solid state imaging device. Therefore, the power consumption of the FIT type CCD camera can be reduced and the FIT type CCD camera can be miniaturized. An image sensor forming a solid state imaging device according to a third embodiment of the present invention is arranged as shown in FIG. 15. As described before, the image sensor comprises an imaging section 111 having sensor sections 118, a storage section 112, a horizontal register 113, a smear drain section 114 and a signal charge detecting section 115. The imaging section 111 and the storage section 112 include vertical registers 116a,b provided at every sensor sections 118 arranged in the vertical direction of the imaging section 111. A transfer driver (not shown) which drives the vertical register 116a is driven by 4-phase clock signals IM 1B , IM 2B , IM 3B and IM 4B . A transfer driver (not shown) which drives the vertical register 116b is similarly driven by 4-phase clock signals ST 1B , ST 2B , ST 3B and ST 4B . A transfer driver (not shown) that drives the horizontal register 113 is driven by clock signals H 1B and  H 2B . The smear drain section 114 is supplied with a clock signal SMG and a DC drain voltage V SMD and whose drain section is opened and closed in response to the signal level of the clock signal SMG. FIG. 16 is a timing chart showing the above-mentioned clock signals IM 1B to IM 4B , ST 1B to ST 4B and the vertical blanking signal V-BLK. As shown in the timing chart of FIG.16. there are provided a line shift transfer 121, a smear discharge transfer 122, a signal charge read-out 123, a frame shift transfer 124 and a line shift transfer 125. In addition, there are provided smear electric charge coarse discharge periods P1, P2 prior to the smear discharge transfer 122. The vertical blanking signal V-BLK controls the smear electric charge coarse discharge period P1, the smear discharge transfer 122, the signal charge read-out 123 and the frame shift transfer 124 when it is in the low level period. As shown in FIG. 16, according to this embodiment, during the period P1, all channel portions of the vertical register 116a in the imaging section 111 are held at low voltage (high electron potential) level. During the period P2, all channel portions of the vertical register 116b in the storage section 112 are held at high voltage (low electron potential) level. FIG. 16 shows the example that the periods P1, P2 proceed at the same time. FIG. 17 shows the condition that electric charges are transferred in the image sensor according to this embodiment in accordance with the timing chart of FIG. 16. As shown in FIG. 17, at a timing point T 11 in which the line shift transfer 121 is ended, a large quantity of smear electric charges 141 produced by the illumination of high brightness or the like are accumulated in the vertical registers 116. At a timing point T 12 provided within the smear electric charge coarse discharge periods P1 and P2, the vertical transfer clocks IM 1B to IM 4B and ST 1B to ST 4B applied to the vertical registers 116 are all held at low level in the imaging section 111 and all held at high level in the storage section 112. With the above-mentioned arrangement, the smear electric charge 141 can be freely moved within the channel of the vertical registers 116 in accordance with the flat potential level. Since the clock signal SMG in the smear drain section 114 also is opened during this period, almost all of a large quantity of smear electric charges are discharged to the smear drain section 114 from the imaging section 111 through the storage section 112 one at a time. At a timing point T 13 in which the succeeding smear discharge and transfer is carried out, it is sufficient to transfer only a small quantity of smear electric charges 141 remaining within the vertical registers 116. Therefore, as shown at the timing point T 13 , even in the high speed discharge and transfer 122, a driver having a large driving capability need not be provided and almost all of smear electric charges can be discharged. Thus, at a timing point T 14 , the remaining smear electric charge 141 is provided by adding the remaining electric charges in the smear discharge and transfer 122 and new smear electric charges that are generated after the smear discharge and transfer 122. Therefore, the amount of such electric charges is extremely small and is considerably small as compared with the amount of smear electric charges 141 provided at the first timing point T 11 . Thereafter, the read-out transfer 123 of new signal charges from the sensor sections is carried out. At a timing point T 15 in which the signal charge read-out transfer 123 is ended, the smear electric charges 142 and new signal charges 143 exist within the vertical register 116a in the imaging section 111. However, the amount of the smear electric charges 142 is extremely small and cannot affect the image sensor substantially. The electric charges 142, 143 thus added are transferred through the next frame shift transfer 124 and the line shift transfer 125 to the signal charge detecting section 115 and an output of the signal charge detecting section 115 is read out as an output of the image sensor. While the vertical transfer clocks are all held at low level in the image section 111 and are all held at high level in the storage section 112 at the timing point T 12 as described above, the present invention is not limited thereto and the following variant also is possible. Specifically, potential levels of the vertical registers 116a,b in the image section 111 and the storage section 112 are made the same as each other with similar effect being achieved. Both of the coarse discharge periods P1 and P2 of the image section 111 and the storage section 112 are not always made the same as each other. For example, the occurrence of the period P1 may be advanced from that of the period P2 or the duration of the period P1 may be made longer than that of the period P2. In this case, it is possible to substantially reduce a ratio of a coarse discharging time in the whole cycle by overlapping the line shift transfer and the period P1, for example. While there are provided the periods P1, P2 in which the potential levels of the vertical registers 116a,b in both the image section 111 and the storage section 112 are made flat as described above, the present invention is not limited thereto the following variant is also possible. Specifically, the potential levels may be made flat only by the image section 111 or only by one portion of the image section 111 and the line shift transfer 121 may be continued in the remaining portion and the storage section 112. According to the solid state imaging device of this embodiment, the period P1 in which at least one portion of the potential level of the vertical register at least in the image section is made flat, is set prior to the high speed smear discharge and transfer, to thereby move or discharge a large quantity of smear electric charges produced within the vertical register in the imaging section one at a time. Therefore, the amount of the smear electric charges 142 is suppressed to be extremely small so that, when an object of high brightness is picked up, unlike the prior art, there is then no problem that the white smear false signal takes place in the upper side of the picture on the monitor picture screen to deteriorate the quality of the picture reproduced on the monitor picture screen. FIG. 18 is a schematic plan view of the image sensor according to a fourth embodiment of the present invention. The layout shown in FIG. 18 is different from that of the image sensor shown in FIG. 15 in that there is provided a first smear drain section 154 corresponding to the smear drain section 114 of the preceding embodiment and that a second smear drain section 157 is disposed adjacent to the end portion of a vertical register 156a in an image section 151. In FIG. 18, reference numeral 152 depicts a storage section, 153 depicts a horizontal shift register, 155 depicts a signal charge detecting section and 158 depicts a sensor section (sensor pixel). FIG. 19 is a timing chart of respective clocks which drive the vertical registers in this image sensor and the vertical blanking signal V-BLK. FIG. 20 shows a schematic structure of sensor taken along the line Y3 - Y4 in FIG. 18 and potential levels at that portion and the electric charge transferred states, similarly to FIG. 17. According to the fourth embodiment of the present invention, during a period of a line shift transfer 161, transfer clocks IM 1C to IM 4C of the vertical registers 156a in the image section 151 are all maintained at high level. The levels of these transfer clocks IM 1C to IM 4C are not always limited to high level and it is sufficient that they are maintained at constant level. Also, the whole period of the line shift transfer 161 is not always maintained at constant level and it is sufficient that the line shift transfer 161 may be maintained at constant level during a predetermined period prior to the smear discharge and transfer 162. As shown in FIG. 20, at a timing T 21 immediately before the line shift transfer 161 is ended, smear electric charges 171 produced in the vertical register 156 in the image section 151 are constantly discharged to the second smear drain section 151 by holding all of the vertical clocks of the image section 151 at the same constant voltage. This processing is carried out during the line shift transfer 161. A smear discharge and transfer 162 is carried out after the line shift transfer 161. At a timing point T 22 , the line shift transfer is ended and the potential levels of both the vertical transfer sections are made flat prior to the discharge and transfer 162. Smear electric charges generated in the smear discharge and transfer 162 are all sequentially discharged by the smear discharge and transfer 162 to the first smear drain side. A timing point T 23 in FIG. 20 shows such condition. The transferred states after the timing point T 23 are similar to those provided after the timing point T 13 in the preceding embodiment and therefore need not be described in detail. According to this embodiment, since the smear electric charge coarse discharge period is sufficiently long, the coarse discharge capability of the smear electric charge can be increased considerably as compared with that of the preceding embodiments. Since the quantity of smear electric charges remaining in the vertical registers in the image section when new signal charges are read out after the discharge and transfer was ended is extremely small as described above, the level of the smear false signal caused by the smear electric charges can be suppressed to be small. In the conventional image sensor, it has been customary to use a bipolar transistor of high driving capability to discharge a large quantity of smear electric charges rapidly as a transfer driver for driving the vertical register. However, in the case of the aforesaid embodiments, since the quantity of smear electric charges to be discharged by transferring is small, the bipolar transistor type driver may be replaced with a CMOS (complementary metal oxide semiconductor) type driver, which can therefore reduce the power consumption considerably. As described above, according to the solid state imaging device of the present invention, since the coarse discharge of smear electric charges is carried out by the application of the clock voltage at the vertical gate so that the potential levels of at least one portion of the vertical register are made flat, the smear electric charges can be reliably discharged and transferred substantially completely even by a small transfer driving force. Therefore, even when an object of high brightness is picked up, the level of the smear false signal generated in the output of the solid state imaging device can be suppressed to be small and an image signal of high quality can be transmitted. Also, it is possible to reduce a power consumption. Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to that precise embodiment and that various changes and modifications could be effected therein by one skilled in the art without departing from the spirit or scope of the invention as defined in the appended claims.

Claims: A method of driving a solid state imaging device (51) which includes an imaging section (54) for converting incident light into a signal charge and transferring said signal charge and an accumulation section (56) for temporarily accumulating said signal charge from said imaging section and transferring the same, said method of driving a solid state imaging device being characterized in that transfer clock pulses (ST) applied to a vertical transfer register (55) in said accumulation section (56) are temporarily stopped to fix said vertical transfer register (55) to a constant potential deeper than a potential of low level of a vertical transfer register (53) in said imaging section (54) to thereby discharge and transfer excess electric charges to a drain region (58). A method of driving a solid state imaging device (51) according to claim 1, wherein transfer clock pulses (ST) applied to said vertical transfer register (55) in said accumulation section (56) are temporarily stopped to fix said vertical transfer regist. (55) to a constant potential deeper than a potential of low level of said vertical transfer register (53) in said imaging section (54) to thereby discharge and transfer excess electric charge in a substantially continuous manner. A method of driving a solid state imaging device which includes: an imaging section (111/151) composed of a plurality of sensor sections (118/158) arranged in horizontal and vertical directions and first vertical transfer sections (116a/156a) disposed in response to every sensor section arranged in said vertical direction and sequentially transfemng signal charges read out from each said sensor section (118/158) in the vertical direction by movement of potential wells; a storage section (112/152) having second vertical transfer sections (116b/156b) disposed in response to said first vertical transfer sections (116a/156a), receiving signal charges transferred from said first vertical transfer sections by movement of said potential wells and temporarily storing said signal charges; a horizontal transfer section (113/153) for reading signal charges transferred thereto from said second vertical transfer sections (116b/156b) at a predetermined timing and transferring the same in the horizontal direction; a signal read-out section (115/155) for reading signal charges sequentially transferred from said horizontal transfer section; and a smear drain section (114/154,157) for discharging unnecessary electric charges of said first and second vertical transfer sections, characterized in that there is provided a coarse unnecessary-electric-charge discharge period in which a potential level of at least one of said first (116a, 156a) and second (116b, 156b) vertical transfer sections is arranged to be flat in the transfer direction by the application of a vertical gate clock voltage. A method according to claim 3, wherein potentials of said first vertical transfer sections (116a/156a) are fixed to be low level and potentials of said second vertical transfer sections (116b/156b) are fixed to be high level during said coarse discharge period respectively. A method according to claim 3, wherein potentials of said first (116a/156a) and second (116b/156b) vertical transfer sections are fixed to be same level as each other during said coarse discharge period. A method according to claim 4, wherein periods or timings at which said potential levels are arranged to be flat in the transfer direction are made different from each other in said first (116a/156a) and second (116b/156b) vertical transfer sections. A method according to claim 3, wherein only the potential level of said first vertical transfer section (116a/156a) is arranged to be flat during said coarse discharge period. A method of driving a solid state imaging device comprising: an imaging section (151) composed of a plurality of sensor sections (158) arranged in honzontal and vertical directions and first vertical transfer sections (156a) disposed in response to every sensor section arranged in said vertical direction and sequentially transferring signal charges read out from each said sensor section in the vertical direction by movement of potential wells; a storage section (152) having second vertical transfer sections (156b) disposed in response to said first vertical transfer sections (156a), receiving signal charges transferred from said first vertical transfer sections (156a) by movement of said potential well and temporarily storing said signal charges; a horizontal transfer section (153) for reading signal charges transferred thereto from said second vertical transfer section (156b) at a predetermined timing and transferring the same in the horizontal direction; a signal read-out section (155) for reading signal charges sequentially transferred from said horizontal transfer section (153); a first smear drain section (154) for discharging unnecessary electric charges from said first and second vertical transfer sections; and a second smear drain section (157) for discharging unnecessary electric charge from said first and second vertical transfer sections; characterized in that there is provided a coarse unnecessary-electric-charge discharge period in which a potential level of at least one of said first (156a) and second (156b) vertical transfer sections is arranged to be flat in the transfer direction by the application of a vertical gate clock voltage. A method according to claim 8, wherein only a potential level of said first vertical transfer section (156a) is made flat during said coarse discharge period. A method according to claim 8, wherein said first and second smear drain sections correspond to said second and first vertical transfer sections, unnecessary electric charges are discharged to said second smear drain section (157) during said coarse discharge period and there is provided a discharge and transfer period in which said first and second vertical transfer sections discharge unnecessary electric charges to said first smear drain section (154). A method according to claim 8, wherein a potential of said first vertical transfer section (156a) is held at low level and a potential of said second vertical transfer section (156b) is held at high level, respectively. A method according to claim 8, wherein potentials of said first (156a) and second (156b) vertical transfer sections are held at the same level as each other during said coarse discharge period. A method according to claim 8, wherein periods or timings at which said potential is held in the transfer direction are made different from each other in said first (156a) and second (156b) vertical transfer sections.

Inventors: IESAKA MAMORU, KUMESAWA TETSURO, IESAKA, MAMORU, KUMESAWA, TETSURO
Applicants: SONY CORP, SONY CORPORATION
Application year: 1994
Main classification: H04N3
IPC codes: H04N3